where to put thumbnails (and how to name)

I am a truly new users, and experimenting with Exhibit Builder plugin.
I've scanned and uploaded a few pictures as pdf files. Now I realize that Omeka can't natively create thumbnails from the pdf files. I can manually do this - but my questions are:
1. where do I place the thumbnails?
2. what do I name them
3. If I get answers to #1 & 2 will that 'automatically' link the thumbnails to the pages in Exhibit Builder